To provide a mobile communication system and a communication method for enabling HARQ control information to be transmitted and received using radio resources efficiently in case that a base station apparatus and a mobile station apparatus communicate with each other in a wider frequency band using component carriers in a composite manner, in the communication method in the mobile station apparatus that transmits, to the base station apparatus, the HARQ control information for a plurality of downlink transport blocks transmitted on a plurality of downlink component carriers using a first physical uplink control channel resource, in case that the mobile station apparatus detects a single physical downlink control channel in a common search space in a certain subframe, the mobile station apparatus transmits the HARQ control information to the base station apparatus, using a second physical uplink control channel resource different from the first physical uplink control channel resource.